The present invention relates to an earth leakage breaking, or ground fault, circuit which detects an earth leakage on an A.C line in a copying apparatus and breaks electrically the A.C line. Furthermore, the present invention relates to a safety circuit of a copying apparatus, and especially relates to a safety circuit which detects a short circuit of an A.C line in a copying apparatus to the copying apparatus frame.
A.C lines in a copying apparatus are naturally connected to an A.C power source which supplies A.C electric power. A D.C power source such as a low tension power source unit of about five volts which drives a control circuit or the like, A.C loads such as a fixing lamp for a fixing unit, and an exposure lamp or the like, and a driver which drives A.C loads are connected to the A.C line.
A plurality of D.C motors, and solenoids or the like as well as a control circuit are connected to the D.C power source, which supplies D.C electric power energy.
As described above, the control circuit is driven by the D.C electric power, and controls a driver to control drive for A.C and D.C loads.
Especially, the load, to which electric power energy is directly supplied from an A.C line, such as a fixing lamp and an exposure lamp, is subject to ignition caused by a ground fault. From the aforesaid background, the inventor of the present invention has developed an earth leakage breaking control circuit which prevents, in advance, such a ground fault accident.
As a means to detect a current difference between A.C lines, there has been widely known a zero-phase current transformer (so-called ZCT), in which coils are wound around A.C lines to provide a detection circuit including the coils. In the means, when there is a current difference between the values of currents incoming and outgoing through the A.C lines, electric potential is generated by electromagnetic induction across both ends of a coil C, and current is obtained through the detection circuit connected to the means.
FIG. 2 is a time chart which shows an output current from a detection circuit, to which a current difference detection means provided on an A.C line in a common copying apparatus is connected.
In FIG. 2, the current which is generated in the current difference detection means and flows through the detection circuit is about 2 to 3 mA, when a ground fault is not caused. The detection circuit detects a large amount of current when electrical loads are started or a power switch is turned on. The case in which the detection circuit detects a large amount of current, except the case of a ground fault, will be described as follows.
The sign t.sub.1 shows that an unbalanced current which is generated when a main switch is turned on, that is, A.C power supply is started on A.C lines, is generated in the current difference detection means and detected by the detection circuit. The unbalanced current, in this case, is not a current caused by a ground fault in an A.C line, but is a transient unbalanced current generated when a capacitor is charged in a D.C power source. In this case, the value of the current generated transiently in the detection circuit is about 150 mA.
During the period from t.sub.1 to t.sub.2, the control circuit conducts an initializing for control and an output from the detection circuit is not inputted into the control circuit. The control circuit begins to drive A.C loads and D.C loads after the point of t.sub.2 and reads in the output from the detection circuit to start processing ground leakage detection.
The time chart in FIG. 2 shows that A.C loads or D.C loads are started at the time of t.sub.3 through t.sub.7, and the detection circuit momentarily detects the current of about 30 mA at each point of time. The current at each time is not always a current due to the ground fault in the A.C line, but is a transient unbalanced current as described above.
In order not to detect the transient current which is not caused by the ground fault unlike the foregoing, it is considered that a capacitor or a timer is provided on an output stage of the detection circuit so that the noise can be absorbed. In this way, however, a response of the circuit after the detection of the ground fault has been deteriorated, resulting in a failure of the sure monitoring of a ground fault and complicated circuit structure.
Furthermore, even if the copying apparatus is interrupted by the start of the earth leakage breaking circuit, the cause of the interruption can not be cleared up, and when the copying apparatus is started again repeatedly, there is the risk of the occurrence of an earth leakage accident.
Generally, a copying apparatus is provided with a plurality of A.C loads such as a heater, a motor, and accordingly is provided with many A.C lines which supply an electric power to the A.C loads.
These A.C lines have the risk of a short circuit (it is called a ground fault, hereinafter) to a copying apparatus body or various grounded portions therein. When the ground fault occurs, the electric potential of the whole copying apparatus frame rises up to that on one end of the A.C power source. Therefore, when a user touches a conductive portion of a copying apparatus external parts, there is the risk of an electric shock. Further, when the ground fault occurs under an unstable condition, an electric arc is caused between a cable of an A.C line and a copying apparatus frame, resulting in a secondary accident such as smoke emission, ignition, and others.
Especially, when there is an ignitable material such as plastics in the vicinity of a place of the ground fault, it is very dangerous.
The result obtained from the experiment shows that the material emits smoke when the ground fault current from the A.C line is 4 to 5 A, and it ignites when the ground fault current is 6 to 7 A.
Therefore, as a measure to counter the above-described secondary accident, it is instructed in an instruction manual and others to ground a copying apparatus frame with a grounding wire. Various measure are taken such as making plastics widely used in a copying apparatus to be flame resisting, reviewing the A.C line for preventing the ground fault, and adding a fuse additionally to the A.C line. Furthermore, it is proposed to provide a household earth leakage circuit breaker on the A.C line.
However, any of the above-described measures to prevent a ground fault in a copying apparatus is not perfect. Regarding the grounding of an apparatus frame, it is not always perfectly conducted depending on the condition of the office where the copying apparatus is installed. Even if a fuse or a breaker is provided on the A.C line, it is not operated by a ground fault with a small amount of current. Therefore it is extremely dangerous, and can not be a perfect measure to prevent a ground fault.